A copper alloy casting used as a water contact metal fitting, friction engaging member, or the structural material thereof (half-finished products) needs to have high machinability and wear resistance as well as the strength and corrosion resistance. However, CAC406, CAC602 and CAC604 of JIS H5120, CAC406C of JIS H5121 or the like, which are generally used as the structural material of said copper alloy casting, are not satisfactory in the above-mentioned properties, castability or the like.
In the related art, it is well known that grain refinement is extremely effective in improving the strength, castability or the like of a copper alloy casting by removing dendrite structure, a typical structure of a casting.
Basically, the grains of a copper alloy are refined as follows: (A) the grains are refined during the melt-solidification of a copper alloy, or (B) the grains are refined by deforming such as rolling or the like, or heating the melt-solidified copper alloy (ingot such as slab or the like; casting such as die casting or the like; molten casting products or the like), in which stacking energy such as distortion energy or the like acts as a driving force. In both cases, Zr is known as an element contributing to the grain refinement effectively.
In the case of method (A), since the grain-refining effect of Zr during melt-solidification is considerably dependent upon the other elements and the contents thereof, the grains as refined as desired cannot be obtained. As a result, method (B) is commonly used, and the grains are refined by heating and then deforming a melt-solidified ingot, casting or the like.
JP-B-38-20467 discloses that the grains are further refined as the content of Zr increases; for instance, it discloses the measurement results that the mean grain size of a copper alloy containing Zr, P, and Ni, on which solution treatment is performed and then cold-working is performed at a working rate of 75%, is 280 μm when no Zr is contained, 170 μm when 0.05 mass % of Zr is contained), 50 when 0.13 mass % of Zr is contained, 29 μm when 0.22 mass % of Zr is contained, and 6 μm when 0.89 mass % of Zr is contained. In addition, JP-B-38-20467 suggests that the content of Zr should be in the range of 0.05 to 0.3 mass % in order to avoid a negative influence caused by the excessive addition of Zr.
Furthermore, JP-A-2004-100042 discloses that the mean grain size can be as fine as about 20 μm or less if a copper alloy, to which 0.15 to 0.5 mass % of Zr is added, is subject to solution treatment and then deformation process after casting.    Patent Document 1: JP-B-38-20467    Patent Document 2: JP-A-2004-100042
However, if a casting is heated and deformed to refine the grains like method (B), the manufacturing cost rises. Also, there is a casting, the shape of which makes a deformation process impossible. Therefore, it is preferable that the grains be refined while a copper alloy is being melt-solidified by method (A). However, in the case of method (A), as described above, since the grain-refining effect of Zr considerably depends on the other elements and the contents thereof during melt-solidification, when the content of Zr increases, it does not necessarily mean that the grains are refined as much as expected by the increased amount. In addition, since Zr has an extremely strong affinity to oxygen, when Zr is melted and added in the air, Zr is likely to be oxidized, and thus the yield of Zr decreases drastically. As a result, even when a manufactured casting contains a little amount of Zr, a large amount of Zr raw material needs to be charged upon pouring. On the other hand, if too much oxide is generated during melting, the oxide can be included into a molten alloy during pouring, thereby inducing casting defects. In order to prevent the oxide from generating, it can be considered that the raw materials are melted and cast under a vacuum or inert gas atmosphere. This method, however, raises the manufacturing cost. Furthermore, since Zr is an expensive element, it is preferable that the adding amount of Zr be suppressed as low as possible from an economic point of view.
Consequently, it is demanded that the content of Zr be made as low as possible and a copper alloy casting, the grains of which are refined at the stage of melt-solidification during casting, be developed.